Evidence for bias in C/ N, δ13C and δ15N values of bulk organic matter, and on environmental interpretation, from a lake sedimentary sequence by pre-analysis acid treatment methods

Abstract

There is a known bias in C/ N, δ 13 C and δ 15 N values of organic matter (OM) due to pre-analysis acid treatment methods. We report here, for the first time, the results of a pre-analysis acid treatment method comparison of measured C/ N, δ 13 C and δ 15 N values in bulk OM from a sedimentary sequence of samples to illustrate this bias. Here we show that acid treatment significantly reduces the accuracy (between method biases) and precision (within method bias) of C/ N, δ 13 C and δ 15 N values of OM, suggesting a differential response of sample OM between methods and sample horizons, and in some cases inefficient removal of inorganic carbon. We show that different methods can significantly influence environmental interpretation in some of our sample horizons (i.e. interpretation of aquatic vs. terrestrial OM source; C 3 vs. C 4 vegetation). Specifically, there are unpredictable and non-linear differences between methods for C/ N values in the range of ∼1–100; δ 13 C values in the range of 0.2–6.8‰ and; δ 15 N values in the range of 0.3–0.7‰. Importantly, these ranges are mostly much greater than the instrument precision (defined as the standard deviation of replicate analysis of standard reference materials; for this study, ±0.5 for C/ N values, ±0.1‰ for δ 13 C values and; ±0.1‰ for δ 15 N). The accuracy and precision of measured C/ N, δ 13 C and δ 15 N values of bulk OM is not just dependent upon environmental variability, but on acid pre-treatment, residual inorganic carbon and organic matter state and composition. Collectively, this makes the correlation between samples prepared in different ways, including those from down-core reconstructions, highly questionable.